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<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <canvas style="border: 1px solid #ccc;" width="600" height="400" id="mycanvas"></canvas>
</body>
<script>
    let canvas = document.getElementById('mycanvas')
    let ctx = canvas.getContext('2d')
    ctx.globalAlpha = 0.5;
  //同心圆
  for (var i = 0; i < 7; i++) {
    ctx.beginPath();
    ctx.arc(75, 75, 10 + 10 * i, 0, Math.PI * 2, true);
    ctx.fill();
  }
    // ctx.fill()
</script>

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